A flywheel assembly is described which is suitable for use as a compact power storage device. The assembly comprises a housing, a flywheel coupled to a shaft which is rotatably mounted in the housing via bearings, a primary chamber which is defined within the housing and contains the flywheel, and a secondary chamber which is defined within the housing and contains the bearings. Furthermore, it includes a molecular pump driven by the shaft for pumping gas molecules from the primary chamber to the secondary chamber. Location of the bearings in a high pressure chamber inhibits off-gassing from the bearing grease. Furthermore, any such off-gassing under load will not increase the gas pressure in the first chamber containing the flywheel as it is separate from the secondary chamber containing the bearings.